If you send say 20 people to Mars, let’s do the math. An average person requires approximately 2 to 3 lb of food per day. 18 months = 6,500 days x 20 people = 131,000 pounds of food, or about 65 tons. You could probably drop the weight significantly by freeze drying it and recycling the water.

In any case, 65 tons isn’t a whole lot - that’s about what, half of a starship payload? Zubrin’s a case for Mars likewise discussed the need to bring all of your food supplies over with you.

Now over many years you could build up enough buy a waste and build a recycling system to start recycling to buy a waste in a greenhouse, but we don’t know how viable like greenhouse on Mars will be for growing food. It’s likely going to have to be more of a grow lab/vertical farm setup. Very energy intensive.